The importance of cellular blood elements, such as red cells, platelets, and white cells in health and disease has been long recognized. Associated with every organic illness is an involvement of blood cells. Over the past 25 years, interest in blood cells has intensified and the new and multidisciplinary science of cell pathology has emerged. Advances in optical and cell separation techniques, tissue culture and in the knowledge of cell function have made possible to categorize diseases and identify the type of blood cells involved. These have provided a sound basis for studies with radiolabeled blood cells enabling radiologists to identify and localize abnormal lesions, deep in the body, by gamma camera imaging. The use of radiolabeled blood cells, also has potential to enable investigators to better understand the in vivo cell kinetics and pathophysiology of human diseases.
In 1976, McAfee and Thakur (J. Nucl. Med. 17, 480-487) observed several radioactive agents and concluded that indium-111 chelated to 8-hydroxyquinoline (oxine) provides a best radioactive tracer for cellular blood elements. Indium-111 (In-111) is a commercially available, cyclotron produced radionuclide. The half-life (67 hours) and the gamma rays (173 kev and 247 kev) of the radionuclide are well suited for in vivo applications and gamma camera imaging. The short half-life minimizes the radiation dose to a patient and the gamma ray energies are efficiently detected by the gamma camera. The chelation of In-111 with oxine provides a neutrally charged compound that passively diffuses through cell membrane and the radioactivity binds to a desired type of blood cell without affecting the cell viability. See Thakur, J. Nucl. Med. 18:1022 (1977). However, indium-111 oxine, has characteristics, that lead to a number of disadvantages.
The insolubility of oxine in aqueous solvents necessitates the chelating agent to be dissolved in ethanol before adding to the .sup.111 InCl.sub.3 solution in acetate buffer and requires the resultant lipid soluble complex to be extracted in a nonpolar solvent. If the complex is not extracted, oxine, in the aqueous system, may form a colloid during storage and prevent the radioactive tracer from diffusing across the cell membrane. The nonpolar solvent containing the extracted complex must be evaporated and the complex dissolved in absolute ethanol before being used for cell labeling. Fifty ul of ethanol is used to dissolve 1 mCi .sup.111 In-oxine, since a large quantity of the solvent may be toxic to cells. Some investigators have considered this volume to be too inconvenient for dispensing into several test tubes containing cell suspensions.
The disadvantages of more serious consequences arise from the fact that .sup.111 In-oxine has only moderate thermal stability. Therefore, when .sup.111 In-oxide is added to plasma the major proportion of .sup.111 In-oxine immediately binds to the protein transferrin and thereby severely inhibits the ability of the agent to label cells. This necessitates the cells to be suspended in a nonplasma medium for efficient incorporation of radioactivity.
Although the suspension of erythrocytes, neutrophils, and lymphocytes in a nonplasma medium for efficient .sup.111 In labeling does not reportedly result in any apparent loss of cell viability, the suspension of human platelets in normal saline severely reduces platelet aggregability and in vivo survival.
Thakur et al. in J. Nucl. Med. 22, 381-385, (1981) recently demonstrated that modified Tyrode's solution is a better medium than normal saline for .sup.111 In platelet labelling. Yet, suspension of human platelets even in this medium reduces their aggregability to 66.+-.15% of those suspended in autologous plasma. This has limited to some extent the clinical use of .sup.111 In labeled platelets. It has been desired to provide a labelling agent which can be used which does not bind blood components such as transferrin and will effectively radioactively label blood cells in plasma to preserve the physiological functions of the cells after labelling.